The present invention was conceived as a means to specifically provide an adequate level of hydraulic isolation between zones in a non-cased horizontal oil well bore. As such, a cost effective method was being sought to install two or more packers in a tubing `string` as a means to shut off zones of high water inflow. However, the device configurations developed to meet the requirements of this particular application may be applied more generally to include many other applications serviced by packers or bridge plugs and indeed by other annular sealing devices such as blowout preventers.
However the invention will be described in the context of downhole packers and bridge plugs.
Within the context of petroleum drilling and completion systems, existing methods to provide hydraulic isolation (sealing) between portions of a well bore or well bore annulus, whether cased or open, may be broadly divided into two types of seal element: 1) bulk expansion (compression set) and, 2) inflatable. Devices employing either of these seal element methods are commonly referred to as either bridge plugs or packers, depending respectively on whether full cross sectional or annular closure is ultimately required. Since closure of an annular space with respect to the device is always required, the term Packer is employed herein to refer generally to all such devices.
In either case the packer must provide sufficient annular clearance to first permit insertion into the well bore to the desired depth or location and a means to subsequently close this annular clearance to effect an adequate degree of sealing against a pressure differential. It is often also desirable to retract or remove these devices without milling or machining.
Packers relying on bulk expansion of the seal element typically employ largely incompressible but highly deformable materials, such as elastomers, as the sealing element or element `stack` where the element is cylindrically or toroidally shaped and is carried on an inner mandrel. U.S. Pat. Nos. 5,819,846 and 4,573,537 are two examples of such devices using an elastomer and ductile metal (non-elastomeric) respectively for the deformable seal element material. The seal is formed by imposing axial compressive displacement of the element, causing the material to incompressibly expand radially to close off the annular region, and after contact with the confining borehole or casing is achieved, to apply sufficient pre-stress to promote sealing. The amount of annular expansion and sealing achievable with elastomers is dependent on several variables but is generally limited by the extrusion gap allowed by the running clearance. The size of annular gap sealable with ductile metals is similarly limited, although for slightly different reasons, and since the deformation is largely irreversible presents a further impediment to retrieval. For either elastomer or ductile metals practically achievable axial seal lengths are short, in the order of a few inches, and therefore sealing on rough surfaces is not readily achievable. This limitation to sealing small clearances with relatively short seal lengths and limited conformability, even for elastomers, tends to preclude using this method for sealing against most open bore hole surfaces. Furthermore, this style of device must usually also provide a means to react axial load, e.g., slips, separate from the sealing element. Such axial loads arise from pressure differentials acting on the sealed area plus loads transmitted by attached or contacting members. The axial loads typically exceed either the frictional or strength capacity of the seal material. This is especially true as the sealed area (hole diameter) is increased. Managing the setting and possible release of the associated anchoring systems adds considerable complexity to these devices with associated cost and reliability implications. Similarly, the degree of complexity, cost and uncertainty is further increased where the application requires axial load reversal as arises when the pressure differential may be in either direction. Both the sealing and mechanical retaining hardware tends to require significant annular space, therefore the maximum internal bore diameter is significantly smaller than the setting diameter.
Devices relying on inflation of the `membrane` seal element employ a generally cylindrical sealing element (visualize hose), capable of expanding radially outward when pressured from the inside with a fluid. The sealing element is carried on a mandrel with end closure means, to contain pressure, and accommodate whatever axial displacement is required during inflation. The sealing element in these devices is typically of composite construction where an elastomer is reinforced by stiffer materials such as fibre strands, wire, cable or metal strips (also commonly referred to as slats). U.S. Pat. No. 4,923,007 is one example of such a device employing axially aligned overlapping metal strips. Pressure containment by these elements relies largely on membrane action. The sealing element may be considerably longer and more conformable than in bulk expansion devices. Inflation packers are therefore most commonly employed for sealing against the open bore hole wall. The inflation material may be either a gas, liquid or `setting` liquid such as cement slurry. Where the inflation material stays fluid, pressure must be continuously maintained to effect a seal. If the device develops a leak after inflating, the sealing function will be lost. To circumvent this weakness a setting liquid may be used, e.g., cement; therefore pressure need only be maintained until sufficient strength is reached. However the device then becomes much more difficult to remove since it cannot be retracted through reverse flow of the inflation fluid. Typically it can only be removed by machining or milling. Similar to the bulk expansion method, the membrane strength of these devices significantly limits the ability to react axial load and the annular space requirements of membrane end seals and mandrel can be quite large. Therefore inflatable packer elements tend to suffer from the same limited axial load and through bore capacities as bulk expansion packer elements.